The invention relates generally to the field of power over local area networks, particularly Ethernet based networks, and more particularly to a method of power utilization by a powered device receiving power over twisted wire pairs.
Power over Ethernet (PoE), in accordance with both IEEE 802.3af-2003 and IEEE 802.3at-2009, each published by the Institute of Electrical and Electronics Engineers, Inc., New York, the entire contents of each of which is incorporated herein by reference, defines delivery of power over a set of 2 twisted wire pairs without disturbing data communication. The aforementioned standards particularly provide for a power sourcing equipment (PSE) and a powered device (PD). The power sourcing equipment is configured to detect the PD by ascertaining a valid signature resistance, and to supply power over the 2 twisted wire pairs only after a valid signature resistance is actually detected.
The HD BaseT Alliance of Beaverton, Oregon has published the HDBaseT Specification Version 1.1.0 which defines a high power standard utilizing twisted wire pair cabling, such as Category 5e (CAT 5e) or Category 6 (CAT 6) structured cabling as defined by ANSI/TIA/EIA-568-A. The specification provides for even higher power than the above mentioned IEEE 802.3at-2009 over each set of 2 pairs, with all 4 pairs utilized for powering, and allows for power over structured communication cabling from any of: a type 1 PSE, denoted hereinafter as a low power PSE, typically meeting the above mentioned IEEE 802.3af standard; a type 2 PSE denoted hereinafter as a medium power PSE, typically meeting the above mentioned IEEE 802.3at standard; a type 3 PSE, denoted hereinafter as a high power PSE, typically meeting the above HDBaseT specification; twin medium power PSEs; and twin high power PSEs.
In the PoE process, a valid device detection is first performed. This detection process identifies whether or not the PSE is connected to a valid PD to ensure that power is not applied to non-PoE capable devices. After a valid PD is discovered, the PSE can optionally perform a power classification. For example, in the IEEE 802.3af standard, the classification phase identifies a power classification of the PD from among a variety of power classes.
After the classification phase is complete, the PSE is arranged to allocate power to the port in accordance with the power classification. Particularly, the PSE is arranged to provide power to a detected PD only if it is determined that enough power is available to be allocated. For a class 1 PD, the PSE allocates 4.0 W. For a class 2 PD, the PSE allocates 7.0 W. For a class 3 PD, the PSE allocates 15.4 W. Although the PSE allocates the above power amounts, the PD is not authorized to consume the entire allocated amount of power. Particularly, a class 1 PD is allowed to consume up to 3.84 W, a class 2 PD is allowed to consume up to 6.49 W and a class 3 PD is allowed to consume up to 12.95 W. The difference between the amount of power allocated by the PSE and the amount of power consumed by the PD accounts for power loss between the PSE and the PD due to the resistance of the data wires and other components. For example, if the PD draws 300 mA of current, a Category 3 (CAT 3) cable exhibiting a resistance of 20Ω over 100 meters will cause a power loss of 1.8 W, as shown in EQ. 1:PL=I2*R  EQ. 1where PL is the power loss, I is the current drawn through the cable and R is the total resistance of the cable.
As shown in EQ. 1, the power loss is proportional to the resistance of the cable. As a result, a cable of a shorter length, which exhibits a lower resistance, will cause a proportionally smaller power loss. For example, a 300 mA current flowing through a CAT 3 cable of 10 meters will meet a resistance of only 2Ω. Therefore, the power loss through the cable will be only 0.18 W, 1.62 W less than the power loss through a 100 meter long cable. Additionally, a cable exhibiting a lower resistance will cause a proportionally lower power loss. For example, a 300 mA current flowing through a CAT 6 cable of 100 meters will meet a resistance of only 7Ω. Therefore, the power loss through the cable will be only 0.63 W, 1.17 W less than the power loss through a 100 meter long CAT 3 cable.
In summary, in many cases the PSE allocates more power than is required by the combination of the PD power draw and the cable loss between the PSE and the PD. One way of utilizing this excess power is by having the PSE determine the actual cable loss, and adding the unutilized power to the PSE's power budget for other PDs. Another way of utilizing this excess power would be by allowing the PD to consume more power than the maximum rating of its class. For example, as described above, for a class 3 PD the PSE allocates an additional 2.45 W of power to compensate for power loss between the PSE and PD. In the event that the cable resistance is such that the maximum power loss is 1 W, the PD will be allowed to consume an additional 1.45 W of power. In order to allow such an increase in power consumption, the PD needs to receive an indication of the amount of additional power available, which would require communication of data between the PSE and the PD, which is not provided by the above mentioned specifications.
U.S. patent application publication US 2010/0182024, published Jul. 22, 2010 to Yu, the entire contents of which is incorporated herein by reference, is addressed to a system and method for measuring a cable resistance in a PoE application. A short circuit module in the PD is designed to produce a short circuit effect upon receipt of a cable resistance detection voltage. The PSE can thus measure the resistance of the cable responsive to the produced short circuit at the PD, determine the cable resistance, and if desired utilize the over-allocated power for other PDs. Unfortunately, in order to enable the PD to benefit from the cable resistance measurement, as described above, the PSE would need to provide such data to the PD. Such an arrangement would necessitate a data communication link between the PSE and the PD, thereby adding cost and complexity, and as indicated above is not provided for by the above mentioned specifications.